Molecular level cleaning by vapor degreasing has found wide acceptance in industry. In fact, molecular level cleaning by vapor degreasing is a preferred method of cleaning precision parts, such as electronics, machined metallic parts, etc., since vapor cleaning leaves virtually no residue on the parts. Generally, vapor degreasing involves the heating of a solvent to its boiling point to generate a vapor layer into which the object to be cleaned is placed. The vapor condenses on the object and subjects the surface to a solvent-flushing action as it flows downward. The solvent-flushing action dissolves the hydrocarbon contaminants and removes them from the object, thereby cleaning it. The liquid drops are then collected in a reservoir and are revaporized, typically through the use of steam-heating coils. Thus, the surface of the object is continually rinsed with fresh solvent.
There are four general types of vapor phase degreasers. The simplest form of a vapor phase degreaser is the straight vapor cycle degreaser which utilizes only the vapor for cleaning. As the parts are lowered into the hot vapor, the vapor condenses on the cold parts and dissolves the surface oils and greases. The oily condensate drops back into the liquid solvent at the base of the tank. The solvent is evaporated continuously to form a vapor blanket. Since the oils are not vaporized, they remain in the bottom of the tank in the form of a sludge. The scrubbing action of the condensing vapor continues until the temperature of the part reaches the temperature of the vapor whereupon condensation ceases, the part appears dry, and it is removed from the degreaser. The time required to reach this point depends on the particular solvent employed, the temperature of the vapor, the weight of the part, its specific heat and the type of contamination material to be removed. This particular vapor phase degreaser does an excellent job of drying parts after aqueous cleaning and before plating and, thus, it is frequently used for this purpose in the jewelry industry. Unfortunately, however, it is not as effective on small, light weight parts because such parts frequently reach the temperature of the vapor before the condensing action has fully cleaned the parts.
A second type of vapor phase degreaser, i.e., the vapor-spray cycle degreaser, is frequently used to solve the problems associated with the straight vapor cycle degreaser. In this vapor-spray cycle degreaser, the part to be cleansed is first placed in the vapor zone as is done in the straight vapor cycle degreaser. A portion of the vapor is condensed by cooling coils and fills a liquid solvent reservoir. This warm liquid solvent is pumped to a spray nozzle that can be used to direct the solvent on the part, washing off surface oils and cooling the part, thereby cleaning by vapor condensation.
The third type of vapor phase degreaser is a liquid-vapor cycle degreaser which has one compartment with warm solvent and another compartment with a vapor zone. This degreaser is particularly useful for heavily soiled parts or for cleaning a basket of small parts that nest together. The fourth type of vapor phase degreaser is the ultrasonic degreaser. Such degreasers are useful for cleaning critical parts. An ultrasonic degreaser has a transducer mounted at the base of the tank which operates in the range of 20 kHz to 40 kHz. The transducer alternately compresses and expands the solvent forming small bubbles which, in turn, cavitate or collapse on the surface of the part. This cavitation phenomenon disrupts the adhering soils, thereby cleaning the part.
Conventional solvents used with the foregoing vapor phase degreasers include trichloroethylene, perchloroethylene, methyl chloroform, methylene chloride, CFC 113, dibromomethane, bromochloromethane, trichlorotrifluoroethane and various hydrochlorofluorocarbons, such as "Genesolve" (manufactured by Allied Chemical). Vapor degreasing techniques employing the foregoing solvents or equivalents thereof are taught in U.S. Pat. No. 3,881,949 which issued on May 6, 1975 to Carl Martin Brock. Unfortunately, however, such solvents are typically on the Clean Air Act list of high ozone depleting chemicals and, thus, they are being phased out of production and/or banned from use in the United States. Thus, there exists a need for a solvent which can be used in place of these banned ozone depleting chemicals in vapor phase degreasers.
U.S. Pat. No. 4,056,403, which issued to Robert J. Cramer, et al. on Nov. 1, 1977, describes a method in which a number of non-regulated ozone depleting chemicals, including n-propyl bromide, are used in cleaning polyurethane foam generating equipment. Cramer, et al. teach a method wherein a solvent composition described therein is used for cleaning a polyurethane foam generating apparatus or a segment thereof. The solvents taught may be periodically injected under pressure through the mixer portion of the foaming apparatus in order to purge it of residual unreacted or partially foam forming materials. Unfortunately, the method described in this patent would be totally ineffective because its composition does not include the appropriate stabilizers necessary to prevent the n-propyl bromide from becoming acid and thereby attacking the metal surfaces which might be placed into the vapor layer.
The use of hot saturated vapors of a liquid halogenated hydrocarbon, including bromochloromethane, is taught in U.S. Pat. No. 4,193,838 which issued to Robert J. Kelly, et al. on Mar. 18, 1990. More particularly, this patent teaches the generation of a pool of hot saturated vapors of a halogenated acyclic hydrocarbon. Pieces of paper stock which have been coated with "hot melt" coatings, such as are used on consumer items and milk cartons, etc., are then placed in the vapor pool and, thereafter, they are agitated. Again, it is noted that this method would be ineffective at cleaning flux and other articles because of the acidic nature of the non-stabilized compound utilized therein which would tend to destroy the object rather than just clean it.
U.S. Pat. No. 5,403,507, which issued to Richard G. Henry on Apr. 4, 1995, discloses a solvent mixture for use in vapor cleaning degreasing. Dibromomethane is used as the principal component. The dibromomethane is mixed with other solvents which are intended to stabilize the dibromomethane and to prevent the solvent mixture from becoming acidic on the release of bromine into the atmosphere. Although the solvent mixture disclosed therein is more stable than either of the solvent mixtures taught in U.S. Pat. Nos. 4,056,403 and 4,193,838, there are still a number of disadvantages associated with the use of dibromomethane which make it unsuitable for use as a solvent in vapor phase degreasers. In fact, the Clean Air Act now lists dibromomethane as an ozone depleting chemical which is banned from use in vapor degreasers or any other cleaning process which results in atmospheric release.
In view of the foregoing, it is readily apparent that there remains a need in the art for a solvent mixture which is suitable for molecular level cleaning of parts without the use of any of the high ozone depleting chemicals that are identified as Class I or Class II materials in the U.S. Federal Register, Vol. 58, No. 236, Friday, Dec. 10, 1993, Rules and bromochloromethane as a potential ozone depleter and possible banning in the U.S. Federal Register 40 CFR Part 82, Vol. 60, No. 145, Pages 38729-38734, Jul. 28, 1995.